1. Field of the Invention
The present invention relates to a digital camera which has a function of shading correction and/or edge enhancement, and an image signal processing method and a recording medium for the same.
2. Description of the Prior Art
In general, when an image is taken using a lens of a camera, a so-called image field edge brightness reduction phenomenon is created which is a (shading) phenomenon that the light amount decreases with a distance from a central point of the image to an edge of the image. In general, the smaller the f-number (=focal length/effective aperture) is, the larger the influence of the image field edge brightness reduction phenomenon is.
In general, in the case of a silver halide film, since a change in output characteristic exhibits a curved profile as the amount of incident light changes, despite a certain degree of the image field edge brightness reduction phenomenon, it is not noticeable in many cases. However, in the case of imaging equipment, such as a digital camera, which uses a CCD (Charge Coupled Device) which comprises a plurality of, e.g., 1.5 to 3 million pixels arranged in the form of array as an image pickup element, an output characteristic of the CCD changes stepwise pixel by pixel in accordance with a change in the amount of incident light, and therefore, a brightness difference of an image is noticeable in many cases because of the image field edge brightness reduction phenomenon. Noting this, imaging equipment which uses a CCD performs correction of an image field edge brightness reduction for correcting a difference between the brightness in a portion in the vicinity of a central point and the brightness in a peripheral portion, namely, shading correction, on each pixel of an image which was taken. A digital camera, in particular, executes shading correction by means of digital image processing, regardless whether a still image is taken or a movie image is taken.
For example, where a peripheral area in which a light amount decreases and a drop rate in the peripheral area are known in advance, multiplication of an inverse number of the drop rate at this position realizes shading correction. After reading image data of an image by a CCD and storing the image data in a predetermined image memory, a correction value is generated using a two-dimensional coordinate and a function, and a peripheral area with the decreased light amount is shading-corrected. In this case, correction values are stored in other memory as a correction table, and multiplication of the image data is executed using each correction value as a coefficient, to thereby correct the decreased light amount in the peripheral area.
In general, as a correction value for shading correction or the like, data which correspond on one-to-one basis to data regarding the respective pixels are necessary for the purpose of accurate correction.
For instance, image data expressed by 2048 pixels in the horizontal direction and 1536 pixels in the vertical direction result in approximately 3.15 million pixels. In order to assign correction values individually to all pixels of such a large quantity of image data corresponding to as many as 3.15 million pixels, when a correction value of eight bits is to be used for each pixel, for example, it is necessary to prepare a correction table whose size is about three MB as the correction values. Further, since a pixel number of a CCD is expected to increase to 4 to 5 million in the near future, a data size of a correction table is expected to swell up even further to as large as 4 to 5 MB.
In addition, when parameters of optical conditions such as zooming and a stop change, more than one data tables are necessary for each one of those parameters, which in turn increases the total volume of data by several folds.
Noting this, to decrease the size of a data table of correction values, one piece of correction data may be set for each block of 4×4 pixels, for instance, to correct block by block. In this case, the size of a data table of correction values is 1/16 of that when correction values are prepared each for each one of pixels.
However, in this case, since correction values sharply change at contours of blocks, depending on the degree of data correction, a step on blocks becomes noticeable in a peripheral area of an image on a screen, which may deteriorate the quality of the image. Hence, in order to maintain an image quality at a constant level, the conventional approaches have no other alternative but to suppress the degree of shading correction, correction of edge resolution reductions or the like to a certain limit.
Further, while the conventional approaches realize shading correction by means of digital image processing as described above, among characteristics of a general lens is an image field edge resolution reduction phenomenon that a resolution in a peripheral area of an image becomes lower than a resolution in a central area of the image, and the phenomenon varies a distribution of an MTF (Modulation Transfer Function). As the image field edge resolution reduction phenomenon has not been so far addressed by any digital image processing, the image shows increasingly blurred with a distance toward the peripheral area of the image from the central area of the image, and therefore, the quality of the image is poor.